The present invention relates to vapor phase deposition, and more particularly to vapor phase deposition of coatings.
Uniform and monolayer coatings, such as silanes, on silicon based surfaces are desired for a number of applications. In the micromachining of microelectromechanical system (MEMS), a final hydrophobic coating on the device is needed to prevent adhesion of adjacent surfaces due to capillary forces in water.
In contrast, a hydrophillic coating is desired for silicon based medical devices, such as filters or capsules, that are in contact with protein solutions to regulate hydrophilicity and minimize unspecific protein adsorption.
Currently, the coating methods most often used typically involve the assembly of a silane "monolayer" onto a silicon surface in an organic solution. It is known that alcohol groups, being hydrophilic and neutral, can drastically reduce protein adsorption on the surface of contact lenses, glass membranes, and porous silica. To assemble a monolayer of alcohol groups onto a silicon filter surface for protein ultrafiltration, one step is to coat silicon with vinyltrichlorosilane (VTS) or .gamma.-glycidoxy-propyltrimethoxysilane (GPTMS), then convert the vinyl or epoxide resulting from the initial coating step to alcohol groups. Typical precursor molecules are alkyltrichlorosilanes (denoted as RSiCl.sub.3) or alkyltrimethoxysilanes (denoted as RSi(OCH.sub.3).sub.3), where R is any desired functional group to be introduced into the coating. However, trichlorosilanes and trimethoxysilanes are very sensitive to moisture. Even trace amounts of water in the organic solution could lead to polymerization. This causes the formation of multilayers with variable thicknesses, and submicron aggregates or islands on the silicon surface. To avoid this polymerization problem, an alternative method involves the use of monochlorosilane, which is incapable of polymerization. However, monochlorosilanes form a less stable coating than alkyltrichlorosilanes or alkyltrimethoxysilanes.
Another method calls for coating the silanes in a high vacuum. This approach, however, is more expensive than solution coating.